The invention relates to a method for the cold start of a free-piston engine that produces hydraulic or electric energy, adapted for a maximum stroke frequency of at least 10 Hz and comprising a stationary cylinder having a combustion room therein and an air inlet connected thereto, a fuel supply and combustion gas outlet and a piston which is freely movable within the cylinder and which limits the combustion room on one side, wherein energy is supplied to the piston to compress air supplied through the inlet by reducing the combustion room, whereafter fuel is injected into a depression in the piston or the stationary cylinder head to allow the fuel-air mixture to ignite by spontaneous combustion. Such method is known from EP-A-0 254 353.
It is well known that diesel engines are generally hard to start, particularly at low temperatures. This is a result of the fact that the ignition of the fuel-air mixture should take place by spontaneous combustion requiring a high temperature. In conventional diesel engines it is not possible to reach this temperature immediately at the first or second compression stroke. This is no problem with diesel engines having a crank shaft controlled piston, because the electric motor driving the crank shaft allows the piston to make a plurality of strokes in rapid succession causing the temperature in the combustion room to rise and shortly thereafter the circumstances are obtained in which ignition by spontaneous combustion can occur. Nonetheless, diesel engines are often equipped with special aids to facilitate the starting procedure. In diesel engines having a pre-combustion chamber there is often provided a spiral filament within the pre-combustion chamber in order to preheat the pre-combustion chamber. In diesel engines having direct injection there are sometimes provisions for injecting, at the start, ether into the combustion room, which is highly inflammable and consequently facilitates the ignition of the fuel-air mixture. Preheating installations for the entire cylinder head are also known.
In diesel engines having a free piston the starting problem is even worse than in diesel engines having a crank shaft piston because it is not possible to make a plurality of strokes in rapid succession. Then, due to the non-occurrence of an ignition within the combustion room, the piston will not make a sufficient expansion stroke in order to bring the piston to the bottom dead centre again in order to make a new compression stroke by means of the hydraulic or electric device. When no ignition occurs one should first complete a special procedure to bring the piston again to the bottom dead centre and during this period the heat produced within the combustion room by the preceding compression stroke is lost again for the greater part by heat removal. The residence time of the piston in the top dead centre is forcibly short in a free-piston engine so that the combustion conditions are unfavourable compared to a crank shaft engine.